U.S. Pat. No. 3,740,366 discloses pressure sensitive adhesives prepared from an aqueous dispersion or solution of a polymer that includes free carboxylic groups and a compatible polyvalent metal compound. The polyvalent metal compound may be a ionic compound, an organo-metallic complex or a metal chelate that is at least partially ionizable or soluble in the dispersing medium or solvent. Use of zinc and zirconium compounds as suitable polyvalent metal compounds is demonstrated in the Examples of the '366 patent. The adhesive compositions of the '366 patent exhibit improved shear resistance while maintaining good tackiness.
Despite the useful properties of the zinc and zirconium crosslinked emulsions taught in the '366 patent, such compounds are not effective in crosslinking polymer emulsions that have a pH in the acid to neutral range. Furthermore, the use of zinc and other heavy metals has become undesirable from an environmental perspective.
There is a need in the art for an inexpensive, effective, one pot, low toxicity crosslinker that can be used over a wide range of pH to cross link carboxyl-substituted latex polymer emulsions through reaction with the carboxyl substituent groups of the polymer.